This is a renewal application of a interdepartmental stroke program at the Massachusetts General Hospital, involving the Departments of Radiology (Neuroradiology), Neurology, Neurosurgery, Pathology (Neuropathology) and Medicine. The major components of the program during the coming year are the following: 1. Studies of cerebral circulation and metabolism in stroke-prone and stroke patients. These studies utilize 133-xenon inhalation and/or positron emission tomographic techniques; whereas with the 133Xe inhalation studies the emphasis will be on the correlation of blood flow and CO2 reactivity with hemodynamic risk factors for ischemic stroke, the positron efforts would be directed toward testing the feasibility of staging the severity of an ischemic stroke lesion and identifying changes in blood flow and metabolism in response to therapy. 2. Experimental studies of factors complicating cerebral hemorrhagic disorders. One of two projects in this area will evaluate factors which may be responsible for hyperperfusion following experimental hemorrhage into the rat caudate nucleus. The other will relate to subarachnoid hemorrhage, examining mechanisms of, and preventive therapy for the morphologic vascular damage seen by scanning electron microscopic examination of vasospastic vessels.